Millions of people in the U.S. and worldwide are exposed to unsafe levels of inorganic arsenic (iAs) in drinking water. These people are at risk of developing cancer and nonmalignant diseases, including diabetes mellitus. Results of our previous population and laboratory studies challenge the existing paradigm that the iAs-induce diabetes is typical of type 2 diabetes mellitus, which is characterized primarily by insulin resistance due to nonresponsiveness of the liver and peripheral tissues to insulin signaling. In fact, we found significant negative correlations between iAs exposure and fasting plasma insulin and insulin resistance, suggesting impairment in pancreatic beta-cell function. We have also shown that the methylated metabolites of iAs play important roles in the diabetogenic effects of iAs exposure. Our population studies have linked the risk of developing diabetes to the production and urinary levels of one of the toxic metabolites of iAs, dimethylarsinite (DMAsIII). Our laboratory experiments have shown that DMAsIII and its metabolic precursor, methylarsonite (MAsIII), inhibit insulin signaling, but are even more potent as inhibitors of glucose-stimulated insulin secretion (GSIS) in pancreatic islets. Notably, the inhibition of insuli secretion by either MAsIII or DMAsIII was reversed by incubating islets in arsenic-free medium. Taken together, these data suggest that pancreatic beta-cells are the primary targets for iAs exposure and that the toxic methylated metabolites of iAs, MAsIII and DMAsIII, play an important role in beta-cell dysfunction. The goal of this project is to characterize the role of MAsIII and DMAsIII in the diabetogenic effects of iAs exposure and to identify molecular mechanisms by which these arsenicals inhibit GSIS in pancreatic beta- cells. We will use both in vivo and in vitro studies to address this goal. We will compare the susceptibility to develop diabetes in C57BL/6 mice which effectively methylate iAs and C57BL/6/As3mt-KO mice with limited capacity to produce MAsIII and DMAsIII. We will use insulinoma (INS-1/832-13) cells and isolated murine and human pancreatic islets to identify molecular mechanisms by which MAsIII and DMAsIII inhibit GSIS. Finally, we will determine whether the diabetogenic effects of in vivo iAs exposure in mice and in vitro MAsIII and DMAsIII exposures in INS-1 cells or pancreatic islets can be reversed by discontinuing the exposure. This is the first project to examine the role of MAsIII and DMAsIII in the diabetogenic effects of iAs exposure in the endocrine pancreas and to identify the mechanisms by which these metabolites of iAs inhibit pancreatic beta-cell function. Results of this work will help to better understand the phenotype of diabetes associated with iAs exposure. Additionally, data generated by this project will help to improve the diagnosis and treatment of diabetes in populations chronically exposed to environmental or occupational iAs.